travis: added 2.1.0, ruby-head, jruby-head; allow head versions to fail
we should not care about output of resolve!
spec for resolving bin only once
extracted locking for resolving
extracted capture_output
moved BinNotFoundError inside bin_resolver as it is used only there
renamed var bin => name
checking bin versions, closes #33
Merge branch 'checking-bad-bin-versions'
Merge branch 'new-rubies'