Cache pip for travis and appveyor

Updating wheels package since caching was introduced in v7 and I'm not sure what version is default on travis.

.travis.yml

@@ -7,17 +7,28 @@ python:
   - "3.6"
   - "pypy"
   - "pypy3"
+
 branches:
   only:
     - master
+
+cache:
+  directories:
+    - $HOME/.cache/pip
+before_cache:
+  - rm -f $HOME/.cache/pip/log/debug.log
+
 install:
   - pip install coveralls
+  - pip install -U wheels
   - pip install -r requirements.txt
   - pip install -r requirements-dev.txt
   - if [[ $TRAVIS_PYTHON_VERSION != 'pypy'* ]]; then pip install ujson; fi
+
 script:
   - nosetests -v --with-flaky --no-flaky-report --with-coverage --cover-package=telegram/ tests
   - if [[ $TRAVIS_PYTHON_VERSION == 3.5 ]]; then pre-commit run --all-files; fi
   - python ./setup.py bdist_dumb
+
 after_success:
   coveralls

appveyor.yml

@@ -15,12 +15,14 @@ environment:
 install:
   # We need wheel installed to build wheels
   - "git submodule update --init --recursive"
-  - "%PYTHON%\\python.exe -m pip install wheel"
+  - "%PYTHON%\\python.exe -m pip install -U wheel"
   - "%PYTHON%\\python.exe -m pip install -r requirements.txt"
   - "%PYTHON%\\python.exe -m pip install -r requirements-dev.txt"
 
 build: off
 
+cache: C:\Users\appveyor\pip\wheels
+
 test_script:
   - "%python%\\Scripts\\nosetests -v --with-flaky --no-flaky-report tests"