Adapt gitian build script to 0.17 version

0.17 builds on bionic, not on trusty anymore.

Signed-off-by: Cornelius Schumacher <cornelius@thirdhash.com>

contrib/gitian-build.py

@@ -198,7 +198,7 @@ def setup(args):
         subprocess.check_call(['git', 'clone', 'https://github.com/devrandom/gitian-builder.git'])
     if not Path(args.git_dir).is_dir():
         subprocess.check_call(['git', 'clone', args.url, args.git_dir])
-    make_image_prog = ['bin/make-base-vm', '--suite', 'trusty', '--arch', 'amd64']
+    make_image_prog = ['bin/make-base-vm', '--suite', 'bionic', '--arch', 'amd64']
     if args.docker:
         make_image_prog += ['--docker']
     elif not args.kvm: