CI: run travis-ci builds on push only, skip pull requests

[wt: we have few pull reqs and they're automatically closed, however
 seeing them pop up here and there needlessly was adding noise to the
 reports]

.travis.yml

@@ -39,17 +39,17 @@ matrix:
     env: TARGET=linux-glibc OPENSSL_VERSION=1.0.2u
   - os: linux
     arch: amd64
-    if: type != cron
+    if: type == push
     compiler: clang
     env: TARGET=linux-glibc OPENSSL_VERSION=1.1.1d CC=clang-9
   - os: linux
     arch: arm64
-    if: type != cron
+    if: type == push
     compiler: clang
     env: TARGET=linux-glibc OPENSSL_VERSION=1.1.1d CC=clang-9
   - os: linux
     arch: s390x
-    if: type != cron
+    if: type == push
     compiler: gcc
     env: TARGET=linux-glibc OPENSSL_VERSION=1.1.1d
   - os: linux
@@ -67,7 +67,7 @@ matrix:
     compiler: clang
     env: TARGET=linux-glibc OPENSSL_VERSION=1.1.0l FIFTYONEDEGREES_SRC="contrib/51d/src/trie" CC=clang-9
   - os: linux
-    if: type != cron
+    if: type == push
     compiler: clang
     env: TARGET=linux-glibc LIBRESSL_VERSION=3.0.2 CC=clang-9
   - os: linux
@@ -83,11 +83,11 @@ matrix:
     compiler: clang
     env: TARGET=linux-glibc BORINGSSL=yes
   - os: linux
-    if: type != cron
+    if: type == push
     compiler: clang
     env: TARGET=linux-glibc FLAGS= CC=clang-9
   - os: osx
-    if: type != cron
+    if: type == push
     compiler: clang
     env: TARGET=osx FLAGS="USE_OPENSSL=1" OPENSSL_VERSION=1.1.1d
   - os: linux