[Fix #919] Remove auto-correct avoidance in HashSyntax

We have an auto-correct loop that deals with clobbering errors now, so
instead of avoiding a correction we just make the correction range a
bit larger.
Remove a couple of redundant spec examples
Refine a couple of specs
Add auto-correction to NonNilCheck cop
Merge pull request #920 from jonas054/fix_only_hash_syntax_autocorrect

[Fix #919] Remove auto-correct avoidance in HashSyntax
Rename FavorJoin to ArrayJoin
FavorUnlessOverNegatedIf -> NegatedIf, FavorUntilOverNegatedWhile -> NegatedWhile
Fix floats handling in NumericLiterals
Fix an ad-hoc code
[Fix #856] New cop SpaceBeforeFirstArg
New cop SingleSpaceBeforeFirstArg
Remove unneeded Array#uniq in TargetFinder#target_files_in_dir

It's done in #find.
Add option --force-exclusion to always exclude files specified in config

This closes #893.
add link for tips on installing rbx on OSX
Merge pull request #923 from fshowalter/add_rbx_install_link_to_contributing_doc

add link for tips on installing rbx on OSX
Merge pull request #921 from jonas054/space_before_1st_arg

Space before first arg
Merge pull request #922 from yujinakayama/force-exclusion

Add option --force-exclusion to always exclude files specified in config `Exclude`
Fall back to Ruby 2.1 parser if current Ruby runtime is not supported
Enable ruby-head build on Travis CI
Merge pull request #924 from yujinakayama/fallback-parser

Fall back to Ruby 2.1 parser if the current Ruby runtime is not supported in Parser