Don't always build code coverage

.travis.yml

@@ -41,6 +41,7 @@ script:
 after_script: set +e
 after_success: |
     # Build test coverage statistics
+    if [ -z "$CODECOV" ]; then
         wget https://github.com/SimonKagstrom/kcov/archive/master.tar.gz && \
         tar xzf master.tar.gz && \
         cd kcov-master && \
@@ -56,6 +57,7 @@ after_success: |
         done && \
         bash <(curl -s https://codecov.io/bash) && \
         echo "Uploaded code coverage"
+    fi
 
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