Add SHA256 checksum for installer (#504)

* Add SHA256 checksum for installer

* Fix Travis

.travis.yml

@@ -24,6 +24,7 @@ before_install:
 before_script:
   - tree -h
   - export
+  - pushd bin; sha256sum -c install.sha256; popd
   - fish $TRAVIS_BUILD_DIR/bin/install --offline --noninteractive --yes
 
 script:

README.md

@@ -30,6 +30,12 @@ curl -L https://get.oh-my.fish > install
 fish install --path=~/.local/share/omf --config=~/.config/omf
 ```
 
+You can verify the integrity of the downloaded installer by verifying the script against [this checksum](bin/install.sha256):
+
+```
+f2e24da717a9399ebb6a2e78f16627b99b6a0c31aba9123542472de94d9d6da6  install
+```
+
 You can also install Oh My Fish with Git or with an offline source tarball downloaded from the [releases page][releases]:
 
 ```fish

bin/install.sha256

@@ -0,0 +1 @@
+f2e24da717a9399ebb6a2e78f16627b99b6a0c31aba9123542472de94d9d6da6  install