new-lines: refactor to reduce duplicate code

Both options where using identical code.  Now the newline character
is determined beforehand depending on the selected option and then
the same code can be used for all options

yamllint/rules/new_lines.py

@@ -41,13 +41,13 @@
 
 
 def check(conf, line):
+    if conf['type'] == 'unix':
+        newline_char = '\n'
+    elif conf['type'] == 'dos':
+        newline_char = '\r\n'
+
     if line.start == 0 and len(line.buffer) > line.end:
-        if conf['type'] == 'dos':
-            if (line.end + 2 > len(line.buffer) or
-                    line.buffer[line.end:line.end + 2] != '\r\n'):
-                yield LintProblem(1, line.end - line.start + 1,
-                                  'wrong new line character: expected \\r\\n')
-        else:
-            if line.buffer[line.end] != '\n':
-                yield LintProblem(1, line.end - line.start + 1,
-                                  'wrong new line character: expected \\n')
+        if line.buffer[line.end:line.end + len(newline_char)] != newline_char:
+            yield LintProblem(1, line.end - line.start + 1,
+                              'wrong new line character: expected {}'
+                              .format(repr(newline_char).strip('\'')))