snap/snapcraft: pin to v1.1.5 release

[mitchdz]

Instead of using source: . we can pull the source from github. This has the following benefits:

• allow us to set the source-tag. This pulls the gtop source into a temp directory and checks out the tag. This means we don't have to worry about dirty builds.
• The snap name will automatically have ${SNAPCRAFT_PROJECT_VERSION} in it instead of the git hash when using version: git.

To update the snap build, all you will need to do is update version: v1.1.5 to the tag that you want to build.