BUILD: travis-ci: trigger non-mainstream configurations only on daily builds.

Let us save some electricity of Travis-CI.

The following configurations are built on every push:

  - linux-glibc with threads+openssl 1.1.1 on x86 + clang
  - linux-glibc with threads+libressl 2.9.2 on x86 + clang
  - linux-glibc without SSL nor threads on x86 + clang
  - osx with openssl-1.1.1 + clang

The following configurations are built daily:

  - linux-ppc64le + openssl-1.0.2
  - linux + openssl-1.1.0 + 51d/tree
  - linux + libressl-2.8.3
  - linux + libressl-2.7.5 + prometheus
  - linux + boringssl
  - cygwin
  - coverity scan
diff --git a/.travis.yml b/.travis.yml
index bf4e9d3..a656624 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -22,14 +22,10 @@
 
 matrix:
   include:
-  - os: linux
-    if: type != cron
-    compiler: gcc
-    env: TARGET=linux-glibc FIFTYONEDEGREES_SRC="contrib/51d/src/trie"
   - os: linux-ppc64le
-    if: type != cron
+    if: type == cron
     compiler: gcc
-    env: TARGET=linux-glibc OPENSSL_VERSION=1.1.1c LABEL="linux-ppc64le"
+    env: TARGET=linux-glibc OPENSSL_VERSION=1.0.2r LABEL="linux-ppc64le"
   - os: linux
     if: type != cron
     compiler: clang
@@ -45,27 +41,23 @@
           curl -s "https://scan.coverity.com/scripts/travisci_build_coverity_scan.sh" | bash || true
         fi
   - os: linux
-    if: type != cron
-    compiler: clang
-    env: TARGET=linux-glibc OPENSSL_VERSION=1.1.0j
-  - os: linux
-    if: type != cron
+    if: type == cron
     compiler: clang
-    env: TARGET=linux-glibc OPENSSL_VERSION=1.0.2r
+    env: TARGET=linux-glibc OPENSSL_VERSION=1.1.0j FIFTYONEDEGREES_SRC="contrib/51d/src/trie"
   - os: linux
     if: type != cron
     compiler: clang
     env: TARGET=linux-glibc LIBRESSL_VERSION=2.9.2
   - os: linux
-    if: type != cron
+    if: type == cron
     compiler: clang
     env: TARGET=linux-glibc LIBRESSL_VERSION=2.8.3
   - os: linux
-    if: type != cron
+    if: type == cron
     compiler: clang
     env: TARGET=linux-glibc LIBRESSL_VERSION=2.7.5 EXTRA_OBJS="contrib/prometheus-exporter/service-prometheus.o"
   - os: linux
-    if: type != cron
+    if: type == cron
     compiler: clang
     env: TARGET=linux-glibc BORINGSSL=yes
   - os: linux
@@ -77,7 +69,7 @@
     compiler: clang
     env: TARGET=osx FLAGS="USE_OPENSSL=1" OPENSSL_VERSION=1.1.1c
   - os: windows
-    if: type != cron
+    if: type == cron
     install:
       - choco install bash make libssl-devel cygwin-devel gcc-core libgcc1 binutils lua-devel libpcre-devel zlib-devel --source cygwin
     script: