Greenlet on Old Systems

I ran into an issue when trying to set up a Docker container for Plivo; this has to do mostly with greenlet on old systems as well as any packages such as gevent which rely on this.

More specifically, for Python 2.7.3 and older only, projects requiring greenlet may throw an error on attempting to run them

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ImportError: greenlet.so: undefined symbol: _PyTrash_thread_deposit_object

since _PyTrash_thread_deposit_object was not defined in those versions.

This specifically affected me since Plivo requires libgnutls-dev, which is not present in the default debian:jessie or newer base Docker images... using debian:wheezy, though, only gives us Python 2.7.3. Fortunately, this is fixeable without installing anything from non-standard sources!

The issue arises when we pip install greenlet (or pip install PACKAGE, where PACKAGE relies on greenlet, eg. gevent). The simple fix for this is to make sure we don't install the broken greenlet binary:

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pip install --no-binary greenlet gevent

Specifically for Plivo, modifying the plivo_install.sh script with the above change is enough to let us start Plivo with no issues.