Initialize the postgres cluster with no locale support

We change the call to 'initdb' to add --locale=C in order to disable
locale support so that log messages that will then be emitted by
postgres will be in English. Otherwise, initdb will pick the locale
setting of its execution environment [1] which, when not an English one,
will then make the checks in PostgreSQLExecutor.running() (where the
message "pg_ctl: server is running" is expected) fail.

Fix #315.

[1]: https://www.postgresql.org/docs/current/locale.html

src/pytest_postgresql/executor.py

@@ -137,7 +137,7 @@ def init_directory(self):
         # remove old one if exists first.
         self.clean_directory()
         init_directory = [self.executable, 'initdb', '--pgdata', self.datadir]
-        options = ['--username=%s' % self.user]
+        options = ['--locale=C', '--username=%s' % self.user]
 
         if self.password:
             with tempfile.NamedTemporaryFile() as password_file: