Clean up Travis config (#152)

* One Python 3.7 test is enough.
* The docs and lint tests do not produce coverage.

.travis.yml

@@ -4,8 +4,12 @@ matrix:
     include:
         - python: "2.7"
           env: TOXENV=docs,lint
+          before_install:
+          after_success:
         - python: "3.6"
           env: TOXENV=docs,lint
+          before_install:
+          after_success:
         - python: "2.7"
           env: TOXENV=py27,py27-datetime
         - python: "3.5"
@@ -15,16 +19,14 @@ matrix:
         - python: "3.7"
           env: TOXENV=py37,py37-datetime
           dist: xenial
-        - python: "3.7-dev"
-          env: TOXENV=py37,py37-datetime
-          dist: xenial
         - python: "3.8-dev"
           env: TOXENV=py38,py38-datetime
           dist: xenial
-
+before_install:
+    -  travis_retry pip install -U -c constraints.txt coveralls coverage
 install:
     - travis_retry pip install -U pip setuptools
-    - travis_retry pip install -U -c constraints.txt tox coveralls coverage
+    - travis_retry pip install -U -c constraints.txt tox
 
 script:
     - travis_retry tox