Allow specs to run against a different database host

.travis.yml

@@ -12,5 +12,5 @@ before_install:
   - while ! echo exit | nc localhost 3306; do sleep 5; done # wait mysql to start accepting connections
   - sleep 15
 script:
-  - crystal spec
+  - DATABASE_HOST=localhost crystal spec
   - crystal tool format --check

spec/driver_spec.cr

@@ -12,7 +12,7 @@ describe Driver do
   it "should connect with credentials" do
     with_db do |db|
       db.scalar("SELECT DATABASE()").should be_nil
-      db.scalar("SELECT CURRENT_USER()").should match(/^root@(localhost|%)$/)
+      db.scalar("SELECT CURRENT_USER()").should match(/^root@/)
 
       # ensure user is deleted
       db.exec "GRANT USAGE ON *.* TO crystal_test IDENTIFIED BY 'secret'"
@@ -27,9 +27,9 @@ describe Driver do
       db.exec "FLUSH PRIVILEGES"
     end
 
-    DB.open "mysql://crystal_test:secret@localhost/crystal_mysql_test" do |db|
+    DB.open "mysql://crystal_test:secret@#{database_host}/crystal_mysql_test" do |db|
       db.scalar("SELECT DATABASE()").should eq("crystal_mysql_test")
-      db.scalar("SELECT CURRENT_USER()").should match(/^crystal_test@(localhost|%)$/)
+      db.scalar("SELECT CURRENT_USER()").should match(/^crystal_test@/)
     end
 
     with_db do |db|

spec/spec_helper.cr

@@ -4,5 +4,9 @@ require "../src/mysql"
 include MySql
 
 def db_url(initial_db = nil)
-  "mysql://root@localhost/#{initial_db}"
+  "mysql://root@#{database_host}/#{initial_db}"
+end
+
+def database_host
+  ENV.fetch("DATABASE_HOST", "localhost")
 end