Fix travis failing to fetch keys from the sks keyserver pool

Just use the "subset" pool for now, at least none of those are
currently broken, AFAIK.

.travis.yml

@@ -51,7 +51,7 @@ before_script:
     - if [ -n "$OSX_SDK" -a -f depends/sdk-sources/MacOSX${OSX_SDK}.sdk.tar.gz ]; then tar -C depends/SDKs -xf depends/sdk-sources/MacOSX${OSX_SDK}.sdk.tar.gz; fi
     - make $MAKEJOBS -C depends HOST=$HOST $DEP_OPTS
 script:
-    - if [ "$CHECK_DOC" = 1 -a "$TRAVIS_REPO_SLUG" = "bitcoin/bitcoin" -a "$TRAVIS_PULL_REQUEST" = "false" ]; then while read LINE; do travis_retry gpg --keyserver hkp://pool.sks-keyservers.net --recv-keys $LINE; done < contrib/verify-commits/trusted-keys; fi
+    - if [ "$CHECK_DOC" = 1 -a "$TRAVIS_REPO_SLUG" = "bitcoin/bitcoin" -a "$TRAVIS_PULL_REQUEST" = "false" ]; then while read LINE; do travis_retry gpg --keyserver hkp://subset.pool.sks-keyservers.net --recv-keys $LINE; done < contrib/verify-commits/trusted-keys; fi
     - if [ "$CHECK_DOC" = 1 -a "$TRAVIS_REPO_SLUG" = "bitcoin/bitcoin" -a "$TRAVIS_PULL_REQUEST" = "false" ]; then git fetch --unshallow; fi
     - if [ "$CHECK_DOC" = 1 -a "$TRAVIS_REPO_SLUG" = "bitcoin/bitcoin" -a "$TRAVIS_PULL_REQUEST" = "false" ]; then contrib/verify-commits/verify-commits.sh; fi
     - export TRAVIS_COMMIT_LOG=`git log --format=fuller -1`