Use a consistent version of HTTP client

Declare an explicit dependency on httpclient to stop io.searchbox:jest
from pulling in an old version. No version is declared so that the
Spring Boot plugin will determine the version. This change means that
sagan-common and sagan-site are built and tested against the same
version of httpclient.
Merge pull request #411 from wilkinsona/consistent-httpclient

Use a consistent version of HTTP client