Update Readme for Specs

[keshavbiswa]

rake build and rake spec doesn't work without prepending bundle exec:

keshavbiswa@Keshavs-MacBook-Pro rack-mini-profiler % rake build
rake aborted!
LoadError: cannot load such file -- rspec/core
(See full trace by running task with --trace)

[codecov-commenter]

Codecov Report

Merging #564 (b89f2bb) into master (8d5c5a0) will not change coverage.
The diff coverage is n/a.

❗ Current head b89f2bb differs from pull request most recent head 64c3ed5. Consider uploading reports for the commit 64c3ed5 to get more accurate results

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@@           Coverage Diff           @@
##           master     #564   +/-   ##
=======================================
  Coverage   87.39%   87.39%           
=======================================
  Files          18       18           
  Lines        1269     1269           
=======================================
  Hits         1109     1109           
  Misses        160      160           

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