Don't force to use signing key if the certificate is missing.

This is to take care about issues with remote CI environments like Travis CI.

Signed-off-by: Krzysztof Wilczynski <krzysztof.wilczynski@linux.com>
Rely on Bundler to install needed development dependencies.

Signed-off-by: Krzysztof Wilczynski <krzysztof.wilczynski@linux.com>