Experimental HAXE_VAR (git and prebuilt)
Source defaults.sh on gethaxe.sh
Git 3.1.1 and 3.1.0 + don't test master

 - Try to eliminate CI failures
Document HAXE_VER and reenable other tests

 - Missing correct build for Mac
 - Removed 3.1.1 and 3.1.0, since haxelib issues prevented successful
   builds

[skip ci]
Revert to `$(dirname $0)`

 - `~/travis-hx/setup.hx` would fail with `$PWD`, because `$PWD` expands
   to last changed to directory
Merge pull request #7 from jonasmalacofilho/proper-haxe-ver

Added HAXE_VER