bump version to v1.2.2 (#714)

* bump version to 1.2.1

* update installation table in readme

* break new line

* del mmcv upper bound for old installation

* reminder on latest

* bump version to v1.2.2

mmcv/version.py

 # Copyright (c) Open-MMLab. All rights reserved.
 
-__version__ = '1.2.1'
+__version__ = '1.2.2'
 
 
 def parse_version_info(version_str):