travis: make sure to have latest tag

The Win/Mac build scripts name the binaries using "git describe --tags",
so reproducibility requires git to find the latest tag.
By default, Travis uses depth=50.

.travis.yml

@@ -4,12 +4,16 @@ language: python
 python:
     - 3.6
     - 3.7
+git:
+  depth: false
 addons:
   apt:
     sources:
       - sourceline: 'ppa:tah83/secp256k1'
     packages:
       - libsecp256k1-0
+before_install:
+  - git tag
 install:
   - pip install -r contrib/requirements/requirements-travis.txt
 cache: