do not build for special cases

.travis.yml

@@ -237,24 +237,28 @@ install:
 ################
 
 script:
-  # show OS/compiler version
-  - uname -a
-  - $CXX --version
-
-  # compile
-  - make
-
-  # execute unit tests
-  - test/json_unit "*"
-
-  # check if homebrew works (only checks develop branch)
-  - if [ `which brew` ]; then
-    brew update ;
-    brew tap nlohmann/json ;
-    brew install nlohmann_json --HEAD ;
-    brew test nlohmann_json ;
+  - if [[ "${SPECIAL}" != "" ]]; then
+
+    # show OS/compiler version
+    uname -a
+    $CXX --version
+
+    # compile
+    make
+
+    # execute unit tests
+    test/json_unit "*"
+
+    # check if homebrew works (only checks develop branch)
+    if [ `which brew` ]; then
+      brew update ;
+      brew tap nlohmann/json ;
+      brew install nlohmann_json --HEAD ;
+      brew test nlohmann_json ;
     fi
 
+  fi
+
 #language: cpp
 #
 #dist: trusty