Code coverage: ignore embedded Rails apps

[jeremy]

Addresses misleading coverage churn in #83 and #84.

[codecov-io]

Codecov Report

Merging #85 into master will increase coverage by 1.56%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #85      +/-   ##
==========================================
+ Coverage   96.82%   98.38%   +1.56%     
==========================================
  Files          21       16       -5     
  Lines         598      496     -102     
==========================================
- Hits          579      488      -91     
+ Misses         19        8      -11

Impacted Files	Coverage Δ
lib/roadie-rails.rb
spec/support/have_no_styling_matcher.rb
spec/support/have_styling_matcher.rb
spec/support/rails_app.rb
spec/support/have_selector_matcher.rb

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[Mange]

Thanks!