bin_path checks gem name as well as the availability of named bin.
Altered exec spec to reflect less forgiving bin_path.
Merge branch 'bin_path-respects-gem-name' into 2-0-dev

closes #2653
Remove creating GEM_HOME dir before setting it

Reference to: https://github.com/bundler/bundler/issues/602
Merge pull request #2884 from kiela/2-0-dev

Remove creating GEM_HOME dir before setting it
remove check for Bundler 0.8