Fix Gemfile for Ruby 2.7

[mkanoor]

Ruby 2.7 does not bundle the sync gem needed by rails, that gem has to be listed in the Gemfile.

https://www.ruby-lang.org/en/news/2019/12/25/ruby-2-7-0-released/

The following libraries are no longer bundled gems. Install corresponding gems to use these features.
CMath (cmath gem)
Scanf (scanf gem)
Shell (shell gem)
Synchronizer (sync gem)
ThreadsWait (thwait gem)
E2MM (e2mmap gem)

Without this change rspec fails.

*** LoadError Exception: cannot load such file -- sync