Update GolangCI-lint and lint config

The `algo` parameter for the `unparam` linter has been removed and we
should thus no longer specify it. Also, bumping the GolangCI-lint
version to the latest available minor release.

See: mvdan/unparam@e6a6d1c51b

.golangci.yaml

@@ -105,10 +105,6 @@ linters-settings:
     # with golangci-lint call it on a directory with the changed file.
     check-exported: false
   unparam:
-    # call graph construction algorithm (cha, rta). In general, use cha for libraries,
-    # and rta for programs with main packages. Default is cha.
-    algo: rta
-
     # Inspect exported functions, default is false. Set to true if no external program/library imports your code.
     # XXX: if you enable this setting, unparam will report a lot of false-positives in text editors:
     # if it's called for subdir of a project it can't find external interfaces. All text editor integrations

.travis.yml

@@ -13,7 +13,7 @@ env:
   global:
     - GOOS=linux GOARCH=amd64
     - GO111MODULE=on
-    - GOLANGCI_VERSION="v1.14.0"
+    - GOLANGCI_VERSION="v1.15.0"
 
 matrix:
   # It's ok if our code fails on unstable development versions of Go.