tests(travis): cache stunnel install

Author: Salakar

.travis.yml

@@ -16,11 +16,15 @@ node_js:
   - "13"
 
 before_install:
-  - if [[ ! -f downloads/stunnel-5.54.tar.gz ]]; then wget -O downloads/stunnel-5.54.tar.gz ftp://ftp.stunnel.org/stunnel/archive/5.x/stunnel-5.54.tar.gz; fi
-  - if [[ ! -f ./stunnel-5.54/configure ]]; then tar -xzf downloads/stunnel-5.54.tar.gz; fi
+  - if [[ ! -f stunnel.tar.gz ]]; then wget -O stunnel.tar.gz ftp://ftp.stunnel.org/stunnel/archive/5.x/stunnel-5.54.tar.gz; fi
+  - if [[ ! -f ./stunnel-5.54/configure ]]; then tar -xzf stunnel.tar.gz; fi
   - if [[ ! -f ./stunnel-5.54/src/stunnel ]]; then cd ./stunnel-5.54; ./configure; make; cd ..; fi
   - export PATH="$PATH:$(pwd)/stunnel-5.54/src"
 
+cache:
+  directories:
+    - '$TRAVIS_BUILD_DIR/stunnel-5.54'
+
 after_success: npm run coveralls
 before_script:
   # Add an IPv6 config - see the corresponding Travis issue