Use .get_cached_default_environment() from jedi (#406)

In some situations, pyls can call Document.sys_path _very_ frequently. That
method calls jedi.api.environment.get_default_environment(), which can call
jedi.api.environment.find_system_pythons(). On systems with pyenv and many
python versions installed, this can be expensive, as jedi._compatibilty.which
returns a shim path for python major version. Each found version spawns a
subprocess to check the specific version of the python found at the path (via
creation of a jedi Environment). However, if it's only found the pyenv shim,
when a subprocess is spawned, the shim fails to find the version, and pyenv
spends significant time looking for the requested command in other installed
pythons.

Calling .get_cached_default_environment insures jedi goes through the above
process no more than once per 10 minutes per instance of pyls.

Even on systems without pyenv, calling get_default_environment() directly
results in at least one subprocess invocation, and a new jedi Environment object
for each invocation. As Document.sys_path can be called as often as once per
keypress (depending on the client, though this seems to be the case for
lsp-mode) reducing work on that path provides noticeable improvement in
performance.

pyls/workspace.py

@@ -218,7 +218,7 @@ def sys_path(self):
         path = list(self._extra_sys_path)
 
         # TODO(gatesn): #339 - make better use of jedi environments, they seem pretty powerful
-        environment = jedi.api.environment.get_default_environment()
+        environment = jedi.api.environment.get_cached_default_environment()
         path.extend(environment.get_sys_path())
 
         return path