Issue: Update nodeenv from 0.9.4 to 0.9.5

[bh]

My CI script fails after update to 0.9.5:

$ [foo.app01.local] out: nodeenv --requirement=requirements/node/base.txt --python-virtualenv --jobs=4 --quiet

fails with:

[foo.app01.local] out:   File "/home/foo/.virtualenvs/foo/lib/python3.4/site-packages/nodeenv.py", line 665, in main
[foo.app01.local] out:     create_environment(env_dir, opt)
[foo.app01.local] out:   File "/home/foo/.virtualenvs/foo/lib/python3.4/site-packages/nodeenv.py", line 593, in create_environment
[foo.app01.local] out:     install_activate(env_dir, opt)
[foo.app01.local] out:   File "/home/foo/.virtualenvs/foo/lib/python3.4/site-packages/nodeenv.py", line 570, in install_activate
[foo.app01.local] out:     os.symlink("node", join(bin_dir, "nodejs"))
[foo.app01.local] out: FileExistsError: [Errno 17] File exists: 'node' -> '/home/foo/.virtualenvs/foo/bin/nodejs'
[foo.app01.local] out: make: *** [prod_env] Error 1
[foo.app01.local] out: 

Reverting to 0.9.4:

[foo.app01.local] out: nodeenv --requirement=requirements/node/base.txt --python-virtualenv --jobs=4 --quiet
[foo.app01.local] out:  * Install node.js (0.10.29.b'/usr/bin/python2'
[foo.app01.local] out: .. done.
[foo.app01.local] out:  * Overwriting /home/foo/.virtualenvs/foo/bin/shim with new content
[foo.app01.local] out:  * Appending nodeenv settings to /home/foo/.virtualenvs/foo/bin/activate
[foo.app01.local] out:  * Install node.js packages ... done.
[foo.app01.local] out: 

works fine

[ekalinin]

Yep, os.symlink("node", join(bin_dir, "nodejs")) was added by @jhermann in #76.
Will remove this for now.

[jhermann]

Just guard it with an os.path,exists, this only happens if someone repeatedly writes over an existing env.

[jhermann]

See comment on commit cd1eb2e, the "if" is effectively a NOP right now.