Parallel execution of tests makes log difficult to see due to mixing.

Use `git checkout` instead of `git reset`

With this change, if the checkout fails, it will stop with an error.

The purpose is to avoid deleting working branch history when
developing gem.

Refine `.travis.yml`

Fix "undefined method `Pathname'"; fix #4895

Use `exec` instead of `system` in `minirake` for exit status

* Use `rake` instead of `minirake`.
* Remove `gperf` configuration.

Execution time seems to be reduced by about 15%.

#### Before this patch:

  ```console
  $ ./minirake --foo; echo $?
  invalid option: --foo
  0
  ```

#### After this patch:

  ```console
  $ ./minirake --foo; echo $?
  invalid option: --foo
  1
  ```

Perform `shellquote` on referenced string

`mruby` does not warn like `CRuby` for cases like #4893.
Fix #4890, fix #4891, fix #4893.

`_0` is not numbered parameter

#### Before this patch:

  ```console
  $ bin/mruby rb -e '_0=:l; p ->{_0}.()'
  -e:1:13: _0 is not available
  -e:1:13: syntax error, unexpected $end, expecting '}'
  ```

#### After this patch (same as Ruby):

  ```console
  $ bin/mruby rb -e '_0=:l; p ->{_0}.()'
  :l
  ```

Fix potentially crash in `%n` of `mrb_vformat()` with 64-bit `int`

If `mrb_sym` is smaller than `int`, it is promoted to `int`.

Avoid method in method

Remove unnessesary branches

Fix for `#methods` to include methods that were `undef`

Fix `mruby-bin-debugger` tests; ref d2f2f9db

Simplify `print_backtrace()`

Avoid creating `Data` object that refers `mruby` objects.
Also close #4622 ref #4613

This reverts commit f507ff48.
It makes AppVeyor tests fail.

Remove location info from `Exception#inspect`

Refine output of `mrb_print_error()`

The following improvements are made according to Ruby's behavior:

- Match location number to index.
- Remove duplicate most recent call output.
- Fix that first call is not output when array (unpacked) backtrace.

### Example

  ```ruby
  def a; raise "error!" end
  def b; a end
  begin
    b
  rescue => e
    e.backtrace if ARGV[0] == "unpack"  # unpack backtrace
    raise e
  end
  ```

#### Before this patch:

  ```
  $ bin/mruby example.rb unpack
  trace (most recent call last):
    [0] example.rb:2:in b
    [1] example.rb:1:in a
  example.rb:1: error! (RuntimeError)
  ```

#### After this patch:

  ```
  $ bin/mruby example.rb unpack
  trace (most recent call last):
    [2] example.rb:4
    [1] example.rb:2:in b
  example.rb:1:in a: error! (RuntimeError)
  ```

Fix mruby-io test for mingw32

Merge pull request #4878 from shuujii/remove-unneeded-null-checks-to-struct-backtrace_locationfilename

Remove unneeded null checks to `struct backtrace_location::filename`

`struct backtrace_location` is created only in `each_backtrace()`, and
the `filename` field will never be null (it will be `(unknown)` if null).

Remove module only methods from class

Drop dependencies from `mruby-complex` to some gems

Add "mruby developers" into some gems; Resolve #4709