Updated README to Ruby 2.7.3 (#10621)

* Updated README to Ruby 2.7.3

* Updated PREREQUISITES.md

* Updated ruby version in travis.yml

.travis.yml

@@ -3,7 +3,7 @@ sudo: required
 language: ruby
 
 rvm:
-  - 2.6.6
+  - 2.7.3
 
 services:
   - docker

README.md

@@ -98,9 +98,9 @@ For installation, prerequisites include sqlite3 and rvm. [Click here for a compl
 3. Enter the new **plots2** directory with `cd plots2`.
 4. Set the upstream remote to the original repository url so that git knows where to fetch updates from in future: `git remote add upstream https://github.com/publiclab/plots2.git`
 5. Steps to install gems:
-    * You may need to first run `bundle install` if you have older gems in your environment from previous Rails work. If you get an error message like `Your Ruby version is 2.x.x, but your Gemfile specified 2.6.6` then you need to install the ruby version 2.6.6 using `rvm` or `rbenv`.
-	    * Using **rvm**: `rvm install 2.6.6` followed by `rvm use 2.6.6`
-	    * Using **rbenv**:  `rbenv install 2.6.6` followed by `rbenv local 2.6.6`
+    * You may need to first run `bundle install` if you have older gems in your environment from previous Rails work. If you get an error message like `Your Ruby version is 2.x.x, but your Gemfile specified 2.7.3` then you need to install the ruby version 2.7.3 using `rvm` or `rbenv`.
+	    * Using **rvm**: `rvm install 2.7.3` followed by `rvm use 2.7.3`
+	    * Using **rbenv**:  `rbenv install 2.7.3` followed by `rbenv local 2.7.3`
     * Run this `bundle config set without 'production mysql'` from the rails root folder to set your project to exclude libraries only needed in production.
     * Install gems with `bundle install` from the rails root folder.
 6. Run `cp db/schema.rb.example db/schema.rb` to make a copy of `db/schema.rb.example` in `db/schema.rb`.

doc/PREREQUISITES.md

@@ -24,9 +24,9 @@ Install rvm for Ruby management (http://rvm.io)
 
 **Ubuntu users:** You may need to enable `Run command as a login shell` in Ubuntu's Terminal, under `Edit > Profile Preferences > Title and Command`. Then close the terminal and reopen it. You may also want to run `source ~/.rvm/scripts/rvm` to load RVM.
 
-Then, use RVM to install version 2.6.6 of Ruby. (v1.9.3+ should also work):
+Then, use RVM to install version 2.7.3 of Ruby. (v1.9.3+ should also work):
 
-`rvm install 2.6.6`
+`rvm install 2.7.3`
 
 
 ### Gems with Bundler