fix :SKIP reason message string literals

Author: rhysd

t/clang_format_spec.vim

@@ -330,7 +330,7 @@ describe 'g:clang_format#auto_format'
     end
 
     it 'formats a current buffer on BufWritePre if the value is 1'
-        SKIP because somehow BufWritePre event isn't fired
+        SKIP 'because somehow BufWritePre event isn''t fired'
         let formatted = clang_format#format(1, line('$'))
         doautocmd BufWritePre
         let auto_formatted = join(getline(1, line('$')), "\n")
@@ -353,7 +353,7 @@ describe 'g:clang_format#auto_format_on_insert_leave'
     end
 
     it 'formats a inserted area on InsertLeave if the value is 1'
-        SKIP because somehow InsertEnter and InsertLeave events aren't fired
+        SKIP 'because somehow InsertEnter and InsertLeave events aren''t fired'
         execute 10
         execute 'normal' "iif(1+2)return 4;\<Esc>"
         Expect getline('.') ==# '    if (1 + 2) return 4;'
@@ -375,7 +375,7 @@ describe 'g:clang_format#auto_formatexpr'
     end
 
     it 'formats the text object using gq operator'
-        SKIP because of unknown backslash on formatting too long macros
+        SKIP 'because of unknown backslash on formatting too long macros'
         doautocmd Filetype cpp
         let expected = ClangFormat(1, line('$'))
         normal ggVGgq