Recovered Gradle build with latest release (ver. 4.10.2)

TravisCI
- Changed several apt configurations & build steps for Android

AndroidManifest:
- package name: fmt -> org.fmtlib

Gradle:
- Plugin version 3.1.3. Requires Gradle 4.4+
- Removed `x86` (duplicated with `x86_64`)
- `-std=c++17`. Latest NDK(clang7) should support this
- Missing `repositories` configuration

For some reason, the `repositories` config affects to the error:
"Could not find com.android.tools.lint:lint-gradle:26.1.3."

Recovery of Gradle build. 1 of N
Modified several apt configurationss and order of the build

This reverts commit 07200f44 because
the latter doesn't work with virtualenv correctly.

checks for clashes with other function overloads in fmt with templated parameters on the first or second position in the parameter list

std::back_insert_iterators model the OutputIterator concept but differ considerably in their traits and behavior. In particular the former made compilation to fail when format_to_n is given a back_inserter as first argument. The emulation of an OutputIterator is not perfect due to the behavioural differences of back_insert_iterators (e.g. assignment always implies increment) but good enough to be used within fmt's machinery.

Adapt any string-like type to be used by {fmt} just like the standard string types already supported. The adaption is totally non-intrusive.
Signed-off-by: Daniela Engert <dani@ngrt.de>