Test against more recent versions of Go

Run basic tests for all minor versions since 1.11 and full tests for the last
two most recent versions.

Fix #939

.travis.yml

@@ -31,8 +31,12 @@ matrix:
     - go: 1.11.x
       env: RUN_LONG_TESTS=no
     - go: 1.12.x
-      env: RUN_LONG_TESTS=yes
+      env: RUN_LONG_TESTS=no
     - go: 1.13.x
+      env: RUN_LONG_TESTS=no
+    - go: 1.14.x
+      env: RUN_LONG_TESTS=yes
+    - go: 1.15.x
       env:
         - RUN_LONG_TESTS=yes
         - DEPLOY_BINARIES=yes