.travis.yml: move the esp-open-sdk SDK up a dir

Otherwise, an error occurs when installing TRAVIS_SDK=esp8266
*AND* trying to do TRAVIS_TESTS=docs in the same sub-build,
with an error like
    Warning, treated as error:
    .../icmp.h:77:undecodable source characters, replacing with "?"
due to non-ASCII characters in some header file within the esp-open-sdk

Author: jepler

.travis.yml

@@ -45,8 +45,8 @@ before_script:
   - (! var_search "${TRAVIS_SDK-}" nrf || sudo ports/nrf/drivers/bluetooth/download_ble_stack.sh)
 
   # For huzzah builds
-  - (! var_search "${TRAVIS_SDK-}" esp8266 || (wget https://github.com/jepler/esp-open-sdk/releases/download/2018-06-10/xtensa-lx106-elf-standalone.tar.gz && tar xavf xtensa-lx106-elf-standalone.tar.gz))
-  - if var_search "${TRAVIS_SDK-}" esp8266 ; then PATH=$(readlink -f xtensa-lx106-elf/bin):$PATH; fi
+  - (! var_search "${TRAVIS_SDK-}" esp8266 || (wget https://github.com/jepler/esp-open-sdk/releases/download/2018-06-10/xtensa-lx106-elf-standalone.tar.gz && tar -C .. -xavf xtensa-lx106-elf-standalone.tar.gz))
+  - if var_search "${TRAVIS_SDK-}" esp8266 ; then PATH=$(readlink -f ../xtensa-lx106-elf/bin):$PATH; fi
 
   # For coverage testing (upgrade is used to get latest urllib3 version)
   - ([[ -z "$TRAVIS_TESTS" ]] || sudo apt-get install -y python3-pip)