Merge pull request #269 from mattip/drop-old-pypy

MRG: Drop old PyPy versions from CI testing on osx

Together with adding a new PyPy version in #266, no need to test PyPy versions that cannot support NumPy even on Python2.7. FWIW, [PyPy 5.4 was released](https://morepypy.blogspot.com/2016/08/pypy2-v54-released-incremental.html) in Aug 2016, [5.3 was released](https://morepypy.blogspot.com/2016/06/pypy2-v53-released-major-c-extension.html) in June 2016 which is 42 months ago, the cutoff as per [NEP 29](https://numpy.org/neps/nep-0029-deprecation_policy.html#detailed-description) for CPython 

.travis.yml

@@ -87,42 +87,6 @@ matrix:
       env:
         - MB_PYTHON_VERSION=3.8
         - VENV=venv
-    - os: osx
-      env:
-        - MB_PYTHON_VERSION=pypy-4.0
-        - VENV=venv
-    - os: osx
-      env:
-        - MB_PYTHON_VERSION=pypy-5.0
-        - VENV=venv
-    - os: osx
-      env:
-        - MB_PYTHON_VERSION=pypy-5.1
-        - VENV=venv
-    - os: osx
-      env:
-        - MB_PYTHON_VERSION=pypy-5.3
-        - VENV=venv
-    - os: osx
-      env:
-        - MB_PYTHON_VERSION=pypy-5.4
-        - VENV=venv
-    - os: osx
-      env:
-        - MB_PYTHON_VERSION=pypy-5.6
-        - VENV=venv
-    - os: osx
-      env:
-        - MB_PYTHON_VERSION=pypy-5.7
-        - VENV=venv
-    - os: osx
-      env:
-        - MB_PYTHON_VERSION=pypy-5.8
-        - VENV=venv
-    - os: osx
-      env:
-        - MB_PYTHON_VERSION=pypy-5.9
-        - VENV=venv
     - os: osx
       env:
         - MB_PYTHON_VERSION=pypy-6.0