fix: Gemfile & Gemfile.lock to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-RUBY-RACK-572377

Gemfile

@@ -2,8 +2,8 @@ source 'http://rubygems.org'
 
 ruby '2.6.5'
 
-gem 'sinatra'
-gem 'thin'
+gem 'sinatra', '>= 2.0.0'
+gem 'thin', '>= 1.7.0'
 gem 'maruku'
 gem 'i18n'
 gem 'rack-ssl-enforcer'

Gemfile.lock

@@ -1,23 +1,27 @@
 GEM
   remote: http://rubygems.org/
   specs:
-    daemons (1.2.3)
-    eventmachine (1.2.0.1)
+    daemons (1.3.1)
+    eventmachine (1.2.7)
     i18n (0.7.0)
     maruku (0.7.2)
-    rack (1.6.12)
-    rack-protection (1.5.3)
+    mustermann (1.1.1)
+      ruby2_keywords (~> 0.0.1)
+    rack (2.2.3)
+    rack-protection (2.0.8.1)
       rack
     rack-ssl-enforcer (0.2.9)
-    sinatra (1.4.7)
-      rack (~> 1.5)
-      rack-protection (~> 1.4)
-      tilt (>= 1.3, < 3)
-    thin (1.6.4)
+    ruby2_keywords (0.0.2)
+    sinatra (2.0.8.1)
+      mustermann (~> 1.0)
+      rack (~> 2.0)
+      rack-protection (= 2.0.8.1)
+      tilt (~> 2.0)
+    thin (1.7.2)
       daemons (~> 1.0, >= 1.0.9)
       eventmachine (~> 1.0, >= 1.0.4)
-      rack (~> 1.0)
-    tilt (2.0.4)
+      rack (>= 1, < 3)
+    tilt (2.0.10)
 
 PLATFORMS
   ruby
@@ -26,11 +30,11 @@ DEPENDENCIES
   i18n
   maruku
   rack-ssl-enforcer
-  sinatra
-  thin
+  sinatra (>= 2.0.0)
+  thin (>= 1.7.0)
 
 RUBY VERSION
    ruby 2.6.5p114
 
 BUNDLED WITH
-   1.17.2
+   1.17.3