Travis: Rename TRAVIS_NO_EXPORT to DISABLE_EXPORTERS so its meaning is more obvious

.travis.sh

@@ -2,7 +2,7 @@ function generate()
 {
     OPTIONS="-DASSIMP_WERROR=ON"
 
-    if [ "$TRAVIS_NO_EXPORT" = "YES" ] ; then
+    if [ "$DISABLE_EXPORTERS" = "YES" ] ; then
         OPTIONS="$OPTIONS -DASSIMP_NO_EXPORT=YES"
     else
         OPTIONS="$OPTIONS -DASSIMP_NO_EXPORT=NO"

.travis.yml

@@ -47,7 +47,7 @@ matrix:
   include:
     - os: linux
       compiler: gcc
-      env: TRAVIS_NO_EXPORT=YES ENABLE_COVERALLS=ON
+      env: DISABLE_EXPORTERS=YES ENABLE_COVERALLS=ON
     - os: linux
       compiler: gcc
       env: SHARED_BUILD=ON