Fix helm related float bug when using -set option

[LEQADA]

Description

In the travis.yml file we usually declare a global environment variable COMMIT_HASH as a first 8 characters of the commit hash.
Although often there is at least 1 non-numeric character in first 8 characters of a commit hash, sometimes it happens to be a valid number like **44038821**8fb11dcb6649b03f69d0eb3010260cd1.
Later we create an image with a tag which is the truncated commit hash and put to the Container Registry. Then we use this tag to push an image using Helm.

Because of the bug in Helm --set image.tag="$COMMIT_HASH" will be processed as a valid float type variable and become 4.4038821e+07 and later will end up as an error on Kubernetes

couldn't parse image reference "XXX:4.4038821e+07": invalid reference format.
Error: InvalidImageName

In version 2.9.0 to fix this problem Helm introduced another flag to pass a string called --set-string.

Solution

In this PR I change Helm version to the latest one 2.9.1 and replace --set with --set-string in helm upgrade command