Update Travis to Firefox 54.0 and 52.0.3

Windows uses 52.2.0esr, but we need an unbranded build, and there don't
seem to be unbranded builds of the ESR.

.travis.yml

@@ -10,28 +10,28 @@ env:
     global:
         secure: "NxvkbZ7/Op7BTGQRR3C4q8lLoO29f8WtyNN27NSH7AO3H0vBr1Vp5xO8gn+H2qHEug5HvM+YrZ/xAkNXaZVbOInmBmKVMxqVvdpKp9JM1Amf+gzsXWQphfySvs6iqzyP6cwU/jspdvX/WSakgU5v7PWXxtUIaKxdANt6Rw7W+Pc="
     matrix:
-        - FX_VERSION="54.0b"
-        - FX_VERSION="45.0.2esr"
+        - FX_VERSION="54.0"
+        - FX_VERSION="52.0.3"
 matrix:
     fast_finish: true
     #allow_failures:
     #    - env: FX_CHANNEL="beta"
 notifications:
     email: false
 install:
-    - if [ $FX_VERSION = "45.0.2esr" ]; then
-        wget -O tarball "https://archive.mozilla.org/pub/firefox/releases/45.0.2esr/linux-x86_64/en-US/firefox-45.0.2esr.tar.bz2";
+    - if [ $FX_VERSION = "52.0.3" ]; then
+        wget -O tarball "https://archive.mozilla.org/pub/firefox/tinderbox-builds/mozilla-release-linux64-add-on-devel/1491732920/firefox-52.0.3.en-US.linux-x86_64-add-on-devel.tar.bz2";
       fi
-    - if [ $FX_VERSION = "54.0b" ]; then
-        wget -O tarball "https://archive.mozilla.org/pub/firefox/tinderbox-builds/mozilla-beta-linux64-add-on-devel/1496339244/firefox-54.0.en-US.linux-x86_64-add-on-devel.tar.bz2";
+    - if [ $FX_VERSION = "54.0" ]; then
+        wget -O tarball "https://archive.mozilla.org/pub/firefox/tinderbox-builds/mozilla-release-linux64-add-on-devel/1496944705/firefox-54.0.en-US.linux-x86_64-add-on-devel.tar.bz2";
       fi
     - tar xf tarball
 before_script:
     - export DISPLAY=:99.0
     - sh -e /etc/init.d/xvfb start
     - npm i
     - npm run build
-    - if [ $FX_VERSION = "54.0b" ] &&
+    - if [ $FX_VERSION = "54.0" ] &&
          [ $TRAVIS_REPO_SLUG = "zotero/zotero" ] &&
          [ $TRAVIS_BRANCH = "master" ] &&
          [ $TRAVIS_PULL_REQUEST = "false" ]; then