Add gpg subkey for signing releases on Travis-CI

 Encrypted with help from `travis encrypt-file`
 Use this for automating release process

Author: zbeekman

.gitattributes

@@ -21,11 +21,13 @@ prerequisites/build text
 *.tar binary
 *.gz binary
 *.tgz binary
+*.enc binary
 
 # Prevent dev-ops files from making it into the release archives
 .travis.yml export-ignore
 .pullapprove.yml export-ignore
 .gitattributes export-ignore
 .gitignore export-ignore
 developer-scripts export-ignore
-codecov.yml export-ignore
+codecov.yml export-ignore
+*.enc export-ignore

.travis.yml

@@ -67,6 +67,16 @@ matrix:
             - g++-6
 
 before_install:
+  - |
+    set -o errexit
+    if [[ "$TRAVIS_TAG" ]] && [[ "X$TRAVIS_OS_NAME" = "Xlinux" ]] && $TRAVIS_SECURE_ENV_VARS ; then
+      openssl aes-256-cbc -K $encrypted_ef4535c39461_key -iv $encrypted_ef4535c39461_iv -in subkey-328B3A0E-secret.asc.enc -out ./subkey-328B3A0E-secret.asc -d
+      gpg --allow-secret-key-import --import ./subkey-328B3A0E-secret.asc && rm subkey-328B3A0E-secret.asc
+    fi
+    set +o errexit
+  - unset encrypted_ef4535c39461_key || true
+  - unset encrypted_ef4535c39461_iv || true
+  - rm subkey-328B3A0E-secret.asc{,.enc} || true
   - |
     set -o errexit
     if [[ $TRAVIS ]] && [[ "X$TRAVIS_OS_NAME" = "Xosx" ]]; then