Use codecov to estimate coverage

[alurban]

This PR updates the continuous integration build tests to use codecov for coverage estimates, since it's got more useful features than coveralls.

This is similar to gwdetchar/gwdetchar#302.

cc @duncanmmacleod, @jrsmith02

[duncanmmacleod]

LGTM, pending a successful report.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@c761c00). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #121   +/-   ##
=========================================
  Coverage          ?   54.37%           
=========================================
  Files             ?        7           
  Lines             ?      789           
  Branches          ?        0           
=========================================
  Hits              ?      429           
  Misses            ?      360           
  Partials          ?        0

Flag	Coverage Δ
#Linux	54.37% <ø> (?)
#python27	54.25% <ø> (?)
#python35	53.74% <ø> (?)
#python36	53.74% <ø> (?)
#python37	53.74% <ø> (?)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c761c00...aaaa6ee. Read the comment docs.

[alurban]

Thanks @duncanmmacleod, I'm going to go ahead and merge