Downgrade guardian tests to go1.7.5
Downgrade guardian tests to go1.7.5

   - we suspect that newer go version gives high cpu for the compilation

[#140842935]

Signed-off-by: Konstantinos Karampogias <4e1d2d45ac36f8b1926698716dc48aeccc3981d9@swisscom.com>

cloudfoundry-incubator/guardian-release,cloudfoundry-incubator/guardian-release,cloudfoundry-incubator/guardian-release