Fix version command for prereleases

CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Fixed
 
 - Fixed resolution with bad (empty) releases.
+- Fixed `version` for prereleases.
 
 
 ## [0.8.0] - 2018-04-13

poetry/console/commands/version.py

@@ -102,9 +102,9 @@ patch, minor, major, prepatch, preminor, premajor, prerelease.
             # We bump the prerelease part of the version
             sep = ''
             if not extra_matches[0]:
-                extra_matches[0] = 'alpha'
-                extra_matches[1] = '.0'
-                sep = '-'
+                extra_matches[0] = 'a'
+                extra_matches[1] = '0'
+                sep = ''
             else:
                 if extras.startswith(('.', '_', '-')):
                     sep = extras[0]