[PMM-6756] Added go 1.15.x. (#120)

* [PMM-6756] Added go 1.15.x.

* PMM-6756 Update

Co-authored-by: Andrii Skomorokhov <andrii.skomorokhov@percona.com>

.travis.yml

@@ -5,6 +5,7 @@ services:
   - docker
 
 go:
+  - 1.15.x
   - 1.14.x
   - tip
 
@@ -33,7 +34,7 @@ env:
   global:
     - CODECOV_ENV=PMM_SERVER_IMAGE
     # REVIEWDOG_GITHUB_API_TOKEN
-    - secure: "k/9NslWV/PHwRo+0yzBE6i7VUbGkTx/7UOT4tKS2OEuTVeQOTCllVZPKXuYq1j8oRX99L3KA1CQ3GUvAcX5CPcXjqxBj/9SaAqof4OYxnedBRVAsn0aknD6d8VtmMUM0TTkQWCpSWWvee+Ortto5kteWvNdWahwjdU9Y27nnvg1uMHfXWl22bGllzbXPhKr7C4NedC26dSjq5xjGu0c/67eOsxG8E8jR7FfcrLVXOjZfmWR8zD6bn6VeBiBaU9/MWZ5jVpyQdxlm9we3dZplR86v0DKN7hgaq3gxgeg3YUyTCz/ulieI6M+GbIllXxCnACS7o9Jz+aUMxyrQMHzKa32TKKhw8WaPl5IJCsJS7MJECrmISD3lnes3IX+/eiMvl88SdtGn1EPLEI5IbsWrgRWiA3aAb5R6Naj8evEJqBET0MpSR3XI6PgLLcetruZAcRv1QExh90p0M3F2gen98SqdIMCHmWSCneG8hAXif4iH85WAasJyYi2qHOO+ydWEjs6LmCbhlN18Pe0zkn6dVMtz7UboHc0uboCpf7HFG1G0aODSWlta1+TPG9m4tB3PD4/3QKsGu/4CxXHogxmXDZMjdw2xjgWqw6MPY7TPWMQUP7oL5U5wpVPhTE0S4THt9kk9VhgxihLa3ci53XdJkgQMjUIYLKX9VdaeadMrapg="
+    - secure: "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"
 
   matrix:
     - PMM_SERVER_IMAGE=percona/pmm-server:2.0.0