Travis: Update test dependencies when installing and fix coverage

coveralls 1.9 is incompatible with coverage >= 5.0a5

.travis.yml

@@ -13,6 +13,7 @@ env:
     global:
         - BUILD_RUNTIMES=$HOME/.runtimes
         - PYTHONHASHSEED=8675309
+        - PIP_UPGRADE_STRATEGY=eager
         - CC="ccache gcc"
         - CCACHE_NOCPP2=true
         - CCACHE_SLOPPINESS=file_macro,time_macros,include_file_ctime,include_file_mtime
@@ -136,12 +137,12 @@ jobs:
         # First, the build dependencies (see setup.cfg)
         # so that we don't have to use build isolation and can better use the cache;
         # Note that we can't use -U for cffi and greenlet on PyPy.
-        - &build-gevent-deps pip install -U setuptools wheel && pip install cffi cython greenlet
+        - &build-gevent-deps pip install -U setuptools wheel && pip install -U 'cffi;platform_python_implementation=="CPython"' cython 'greenlet;platform_python_implementation=="CPython"'
         # Next, build the wheel *in place*. This helps ccache, and also lets us cache the configure
         # output (pip install uses a random temporary directory, making this difficult)
         - python setup.py bdist_wheel
         - pip uninstall -y gevent
-        - pip install --no-compile `ls dist/*whl`[test]
+        - pip install -U --no-compile `ls dist/*whl`[test]
       script: ccache -s
       before_script: true
       after_success: true
@@ -179,7 +180,7 @@ jobs:
         - ls -l $BUILD_LIBS $BUILD_LIBS/lib $BUILD_LIBS/include
         - python setup.py bdist_wheel
         - pip uninstall -y gevent
-        - pip install `ls dist/*whl`[test]
+        - pip install -U `ls dist/*whl`[test]
         # Test that we're actually linking
         # to the .so file.
         - objdump -p $G_SITE/gevent/libev/_corecffi*so | grep "NEEDED.*libev.so"

setup.py

@@ -365,8 +365,9 @@ def run_setup(ext_modules):
                 'requests',
 
                 # We don't run coverage on Windows, and pypy can't build it there
-                # anyway (coveralls -> cryptopgraphy -> openssl)
-                'coverage>=5.0a4 ; sys_platform != "win32"',
+                # anyway (coveralls -> cryptopgraphy -> openssl).
+                # As of coverage 5.0a6, coveralls (up to 1.9) won't work at all.
+                'coverage<5.0 ; sys_platform != "win32"',
                 'coveralls>=1.7.0 ; sys_platform != "win32"',
 
                 'futures ; python_version == "2.7"',