coverage: only run coverage for subset of unit tests

- Codecov cannot handle as many coverage reports as we are generating

- as a result, our PR coverage pages have been broken for a while, and
  it's hard to tell people where to enhance their testing in PR reviews.

- Scale back to only running coverage for 3.7 and 2.7 unit tests

- This is *probably* better.  We run the build tests for good measure,
  but we do not need to evaluate them for coverage.  The coverage reports
  are about unit tests.

.travis.yml

@@ -34,7 +34,7 @@ jobs:
       sudo: required
       os: linux
       language: python
-      env: TEST_SUITE=unit
+      env: [ TEST_SUITE=unit, COVERAGE=true ]
     - python: '3.4'
       sudo: required
       os: linux
@@ -54,7 +54,7 @@ jobs:
       sudo: required
       os: linux
       language: python
-      env: TEST_SUITE=unit
+      env: [ TEST_SUITE=unit, COVERAGE=true ]
       addons:
         apt:
           packages:
@@ -80,7 +80,7 @@ jobs:
       env: TEST_SUITE=doc
     - os: osx
       language: generic
-      env: [ TEST_SUITE=unit, PYTHON_VERSION=2.7 ]
+      env: [ TEST_SUITE=unit, PYTHON_VERSION=2.7, COVERAGE=true ]
 # mpich (AutotoolsPackage)
     - stage: 'build tests'
       python: '2.7'