[ci] Fixes CI Build

.travis.yml

@@ -3,3 +3,8 @@ language: crystal
 script:
   - crystal spec
   - crystal tool format --check
+
+addons:
+  apt:
+    packages:
+      - pdftk

spec/pdftk_spec.cr

@@ -3,8 +3,6 @@ require "./spec_helper"
 describe Muse::Dl::Pdftk do
   it "should find pdftk binary" do
     pdftk = Muse::Dl::Pdftk.new
-    if pdftk.binary
-      pdftk.binary.should eq "/usr/sbin/pdftk"
-    end
+    pdftk.ready?.should eq true
   end
 end

src/pdftk.cr

@@ -7,14 +7,15 @@ require "dir"
 module Muse::Dl
   class Pdftk
     PDFTK_BINARY_NAME = "pdftk"
-    @binary = "/usr/sbin/pdftk"
+    @binary : String | Nil
     @tmp_file_path : String
 
-    getter :binary
+    def ready?
+      @binary != nil
+    end
 
     def initialize(tmp_file_path : String = Dir.tempdir)
       @tmp_file_path = tmp_file_path
-
       possible_binary = Process.find_executable(Pdftk::PDFTK_BINARY_NAME)
       if possible_binary
         @binary = possible_binary