Update _version.py (#659)

Change _version.py to 1.1.0.

pydicom/_version.py

@@ -1,6 +1,6 @@
 """Pure python package for DICOM medical file reading and writing."""
 import re
 
-__version__ = '1.1.0.dev0'
+__version__ = '1.1.0'
 __version_info__ = tuple(
     re.match(r'(\d+\.\d+\.\d+).*', __version__).group(1).split('.'))