feat: add Migrator#latest? method

May be useful for Continuous Delivery (e.g. abort when not on the latest migration)

Closes #16

spec/migrate/migrator_spec.cr

@@ -27,6 +27,14 @@ describe Migrate::Migrator do
     end
   end
 
+  describe "#latest?" do
+    context "on fresh DB" do
+      it "returns false" do
+        migrator.latest?.should eq false
+      end
+    end
+  end
+
   describe "#next_version" do
     context "on fresh DB" do
       it "returns 1" do
@@ -65,6 +73,14 @@ describe Migrate::Migrator do
     end
   end
 
+  describe "#latest?" do
+    context "after migrations 1 & 2" do
+      it "returns false" do
+        migrator.latest?.should eq false
+      end
+    end
+  end
+
   describe "#next_version" do
     context "after migrations 1 & 2" do
       it "returns 10" do
@@ -121,6 +137,14 @@ describe Migrate::Migrator do
     end
   end
 
+  describe "#latest?" do
+    context "on the last migration" do
+      it "returns true" do
+        migrator.latest?.should eq true
+      end
+    end
+  end
+
   describe "#redo" do
     it do
       migrator.redo

src/migrate/migrator.cr

@@ -50,6 +50,11 @@ module Migrate
       end
     end
 
+    # Return if current version is the latest one.
+    def latest?
+      next_version.nil?
+    end
+
     # Apply all the migrations from current version to the last one.
     def to_latest
       to(all_versions.last)