Remove redundant backslash

windows_2008_r2.json

 {
   "builders": [{
     "type": "vmware",
-    "iso_url": "http://care.dlservice.microsoft.com//dl/download/7/5/E/75EC4E54-5B02-42D6-8879-D8D3A25FBEF7/7601.17514.101119-1850_x64fre_server_eval_en-us-GRMSXEVAL_EN_DVD.iso",
+    "iso_url": "http://care.dlservice.microsoft.com/dl/download/7/5/E/75EC4E54-5B02-42D6-8879-D8D3A25FBEF7/7601.17514.101119-1850_x64fre_server_eval_en-us-GRMSXEVAL_EN_DVD.iso",
     "iso_checksum_type": "md5",
     "iso_checksum": "4263be2cf3c59177c45085c0a7bc6ca5",
     "headless": true,

windows_2012.json

 {
   "builders": [{
     "type": "vmware",
-    "iso_url": "http://care.dlservice.microsoft.com//dl/download/6/D/A/6DAB58BA-F939-451D-9101-7DE07DC09C03/9200.16384.WIN8_RTM.120725-1247_X64FRE_SERVER_EVAL_EN-US-HRM_SSS_X64FREE_EN-US_DV5.ISO",
+    "iso_url": "http://care.dlservice.microsoft.com/dl/download/6/D/A/6DAB58BA-F939-451D-9101-7DE07DC09C03/9200.16384.WIN8_RTM.120725-1247_X64FRE_SERVER_EVAL_EN-US-HRM_SSS_X64FREE_EN-US_DV5.ISO",
     "iso_checksum_type": "md5",
     "iso_checksum": "8503997171f731d9bd1cb0b0edc31f3d",
     "headless": true,