Always check/adjust the installed versions of setuptools/wheel (#1007)

Previously the pip/setuptools/wheel install step was skipped so long
as Python hadn't just been clean installed (ie so long as not a new app,
emptied cache, Python upgrade, stack change) and pip was the expected
version.

This meant that setuptool/wheel could be the wrong version (or even just
not installed at all), and this would not be corrected.

Now, we now use pip itself to determine whether the installed packages
are up to date, since parsing pip's output is fragile (eg #1003) and
would be tedious given there would be three packages to check.

Unfortunately `get-pip.py` uses `--force-reinstall` which means
performing this step every time is not the no-op it would otherwise be,
but this will be resolved by switching away from `get-pip.py` in the
next commit.

Fixes #1000.
Fixes #1003.
Closes #999.

bin/steps/python

@@ -131,7 +131,6 @@ if [ ! "$SKIP_INSTALL" ]; then
   # Record for future reference.
   echo "$PYTHON_VERSION" > .heroku/python-version
   echo "$STACK" > .heroku/python-stack
-  FRESH_PYTHON=true
 
   hash -r
 fi
@@ -162,13 +161,9 @@ if ! curl -s "${GETPIP_URL}" -o "$GETPIP_PY" &> /dev/null; then
     exit 1
 fi
 
-# If a new Python has been installed or Pip isn't up to date:
-if [ "$FRESH_PYTHON" ] || [[ ! $(pip --version) == *${PIP_VERSION}* ]]; then
+puts-step "Installing pip ${PIP_VERSION}, setuptools ${SETUPTOOLS_VERSION} and wheel ${WHEEL_VERSION}"
 
-  puts-step "Installing pip ${PIP_VERSION}, setuptools ${SETUPTOOLS_VERSION} and wheel ${WHEEL_VERSION}"
-
-  /app/.heroku/python/bin/python "$GETPIP_PY" pip=="${PIP_VERSION}" "setuptools==${SETUPTOOLS_VERSION}" "wheel==${WHEEL_VERSION}" &> /dev/null
-fi
+/app/.heroku/python/bin/python "$GETPIP_PY" pip=="${PIP_VERSION}" "setuptools==${SETUPTOOLS_VERSION}" "wheel==${WHEEL_VERSION}" &> /dev/null
 
 set -e
 hash -r