Use automatic git-based versioning for the snap

Snapcraft supports automatic `git describe` based versioning when the
`version` key is set to `git`.  This gives more information of the
source status of a snap distribution.

Refer-to: Snapcraft.yaml reference - doc - snapcraft.io
<https://forum.snapcraft.io/t/snapcraft-yaml-reference/4276>
Signed-off-by: 林博仁(Buo-ren, Lin) <Buo.Ren.Lin@gmail.com>

snap/snapcraft.yaml

@@ -1,5 +1,5 @@
 name: tesseract
-version: master
+version: git
 summary: open source optical character recognition engine
 description: |
   Tesseract has unicode (UTF-8) support, and can recognize more than 100