Use python2 for YCM post-installation

YouCompleteMe's post-installation script only seems to work with python2
(this is actually mentioned in their docs).

It seems that the sample documentation uses the system default python
(which is no longer always python2), so let's better be explicit about
it.

README.md

@@ -195,7 +195,7 @@ In that case, use `do` option to describe the task to be performed.
 
 ```vim
 Plug 'Shougo/vimproc.vim', { 'do': 'make' }
-Plug 'Valloric/YouCompleteMe', { 'do': './install.py' }
+Plug 'Valloric/YouCompleteMe', { 'do': 'python2 install.py' }
 ```
 
 If you need more control, you can pass a reference to a Vim function that
@@ -208,7 +208,7 @@ function! BuildYCM(info)
   " - status: 'installed', 'updated', or 'unchanged'
   " - force:  set on PlugInstall! or PlugUpdate!
   if a:info.status == 'installed' || a:info.force
-    !./install.py
+    !python2 install.py
   endif
 endfunction