Fixed some things in the build

[djspiewak]

A couple things here:

• Releases will now sign tags (fixes Add releaseVcsSign := true to release settings #1717)
• Coursier is used for dependency management (rather than Ivy)
• Doctest is upgraded (the old version actually broke the classpath, and a bug in Ivy masked the problem)
• Updated sbt-travisci

[codecov-io]

Codecov Report

Merging #1718 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1718   +/-   ##
=======================================
  Coverage   93.88%   93.88%           
=======================================
  Files         246      246           
  Lines        4088     4088           
  Branches      154      155    +1     
=======================================
  Hits         3838     3838           
  Misses        250      250

---

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 1931c93...5e9fb14. Read the comment docs.