Fix searx version

Searx version is useful for external report tool like stats.searx.xyz.
Then is better for spot project to track the upstream tag to avoid conflict
during the next rebase. The new spot tag format is the folowing:
`<last searx tag>+<spot tag>`. I suggest to use the spot tag with the date format
`YYYYMMDD`. A full tag should be `v0.15.0+20190719`.

Ref: #36