Issue 4581 - Update buildtools dependency

ensure_dependencies.py

 #!/usr/bin/env python
 
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 import sys
 import os
 import posixpath
 import re
@@ skipping 337 matching lines @@
                 handle.write(sourcedata)
             if __name__ == '__main__':
                 logging.info('Restarting %s' % target)
                 os.execv(sys.executable, [sys.executable, target] + sys.argv[1:])
             else:
                 logging.warning('Cannot restart %s automatically, please rerun' % target)
 
 
 def _ensure_line_exists(path, pattern):
     with open(path, 'a+') as f:
+        f.seek(0, os.SEEK_SET)
         file_content = [l.strip() for l in f.readlines()]
         if not pattern in file_content:
             file_content.append(pattern)
             f.seek(0, os.SEEK_SET)
             f.truncate()
             for l in file_content:
                 print >>f, l
 
 if __name__ == '__main__':
     logging.basicConfig(format='%(levelname)s: %(message)s', level=logging.INFO)
 
     parser = argparse.ArgumentParser(description='Verify dependencies for a set of repositories, by default the repository of this script.')
     parser.add_argument('repos', metavar='repository', type=str, nargs='*', help='Repository path')
     parser.add_argument('-q', '--quiet', action='store_true', help='Suppress informational output')
     args = parser.parse_args()
 
     if args.quiet:
         logging.disable(logging.INFO)
 
     repos = args.repos
     if not len(repos):
         repos = [os.path.dirname(__file__)]
     for repo in repos:
         resolve_deps(repo)