Run green with a single process under pypy3

On Travis CI, the pypy3 tests may fail due to concurrency issues,
probably because of the usage of (shared) temporary files.

So, limit the number of processes used by green to 1 when pypy3 is used.

This implies that failures for pypy3 are no longer allowed.

.travis.yml

@@ -4,7 +4,14 @@ python:
   - "3.3"
   - "3.4"
   - "3.5"
-  - "pypy3"
+env:
+  - GREEN_OPTS=
+
+matrix:
+  include:
+    - python: "pypy3"
+      env: GREEN_OPTS='--processes 1'
+
 install:
   - "python -m pip install pip --upgrade"
   - "pip install ."
@@ -15,7 +22,7 @@ install:
   - "pip install pylint"
   - "pip install codecov"
 script:
-  - "green -vvr"
+  - "green -vvr $GREEN_OPTS"
   - "python -m pylint --errors-only topydo test"
 # Cache Dependencies
 after_script: