travis: install krb5-user only for --with-gssapi build

Reduces the time needed for the other jobs a little.

Closes https://github.com/curl/curl/pull/3721

.travis.yml

@@ -31,7 +31,6 @@ addons:
             - libidn2-0-dev
             - libssh2-1-dev
             - libssh-dev
-            - krb5-user
             - autopoint  # for libpsl that needs autoreconf that uses gettext that needs it
             - libunistring-dev # for libidn2 needed by libpsl
             - libnss3-dev
@@ -46,6 +45,13 @@ matrix:
           env:
               - T=normal C="--with-gssapi --with-libssh2" CHECKSRC=1
               - OVERRIDE_CC="CC=gcc-8" OVERRIDE_CXX="CXX=g++-8"
+          addons:
+              apt:
+                  sources:
+                      - *common_sources
+                  packages:
+                      - *common_packages
+                      - krb5-user
         - os: linux
           compiler: gcc
           dist: trusty