Add String#freeze test

Use #nil? instead of == nil.

Refactor mrubygem code (range.rb, numeric.rb, string.rb, array.rb, enum.rb)

numeric.rb: refactor code (Avoid using {...} for multi-line blocks,  Surrounding space missing in default value assignment)

range.rb: refactor code (use ! instead of not, use favor modifier if and unless usage when having a single-line body)

array.rb: refactor some code

irep dump: fix format string for MRB_USE_FLOAT

delete duplicate definition of Exception.exception

It overwrote the original definition in src/error.c, line 446.

IEC 60559 single format has 6 to 9 significant decimal digits precision.

However the printf conversion specifier e (and E, of course) already
writes 1 digit - the one before the decimal point - and precision
specifies the number of digits to write after the decimal point.

Remove duplicated RARRAY_LEN check in ary_elt

Remove non-need tmp value in mrb_obj_to_sym

Add build on Trusty and OS X

The default 6.1 was slow

be able to source with :path for mrbgems