Ubuntu解決virtualenv升級錯誤

今天爲了安裝一個開源軟件，需要升級virtualenv
於是我運行了官方給我的命令運行

pip install --upgrade virtualenv
virtualenv env

沒想到報錯了，錯誤如下

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/pkg_resources.py", line 444, in _build_master
    ws.require(__requires__)
  File "/usr/lib/python3/dist-packages/pkg_resources.py", line 725, in require
    needed = self.resolve(parse_requirements(requirements))
  File "/usr/lib/python3/dist-packages/pkg_resources.py", line 632, in resolve
    raise VersionConflict(dist,req) # XXX put more info here
pkg_resources.VersionConflict: (virtualenv 15.0.0 (/usr/local/lib/python3.4/dist-packages), Requirement.parse('virtualenv==13.1.2'))

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/bin/virtualenv", line 5, in <module>
    from pkg_resources import load_entry_point
  File "/usr/lib/python3/dist-packages/pkg_resources.py", line 2749, in <module>
    working_set = WorkingSet._build_master()
  File "/usr/lib/python3/dist-packages/pkg_resources.py", line 446, in _build_master
    return cls._build_from_requirements(__requires__)
  File "/usr/lib/python3/dist-packages/pkg_resources.py", line 459, in _build_from_requirements
    dists = ws.resolve(reqs, Environment())
  File "/usr/lib/python3/dist-packages/pkg_resources.py", line 628, in resolve
    raise DistributionNotFound(req)
pkg_resources.DistributionNotFound: virtualenv==13.1.2

從錯誤提示可以看出，virtualenv版本衝突了。可是我們剛纔不是已經更新了virtualenv的版本了嗎。於是谷歌，找到一個答案說需要安裝python3，並將系統默認的python指向它。照做了，然而還是沒用。
後來想到，一開始我安裝virtualenv的時候，用的不是pip，而是easy_install，我簡直是自己坑自己。於是，我運行了下面的命令。

easy_install --upgrade virtualenv

再嘗試了下，嗯問題解決