Change testrpc path for yarn compatibility

[cgewecke]

Verified that it runs without issue on zeppelin (which installs it own testrpc too). Resolves #45

[codecov-io]

Codecov Report

Merging #48 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #48   +/-   ##
=======================================
  Coverage   97.56%   97.56%           
=======================================
  Files           6        6           
  Lines         329      329           
  Branches       78       78           
=======================================
  Hits          321      321           
  Misses          8        8

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