Unbreak Travis-CI build for dev-esp32 branch (#2086)

Changes:
- now forcing use of the OS python version, rather than new "magic" versions in /opt
- installing GNU make 4.1, since our build depends on that these days

.travis.yml

@@ -1,16 +1,21 @@
-sudo: false
+sudo: required # due to manual install of make 4.1
+dist: trusty # due to the make 4.1 not installable on precise
 language: cpp
+# we need at least GNU Make 4.0, which isn't available in trusty, so grab it
+# manually from xenial
+before_install:
+- wget http://mirrors.kernel.org/ubuntu/pool/main/m/make-dfsg/make_4.1-6_amd64.deb
+- sudo dpkg -i make_4.1-6_amd64.deb
 addons:
   apt:
     packages:
     - python-serial
-    - srecord
     - gperf
-cache:
+# note that we clobber $PATH completely, to get rid of /opt/python* stuff so
-  directories:
+# we can use the system python and python-serial, rather than messing around
-    - cache
+# with pip all over the place
 install:
-  - export PATH=$PWD/tools/toolchains/esp8266/bin:$PWD/tools/toolchains/esp32/bin:$PATH
+  - export PATH=$PWD/tools/toolchains/esp8266/bin:$PWD/tools/toolchains/esp32/bin:/bin:/usr/bin:/sbin:/usr/sbin
 script:
 - export BUILD_DATE=$(date +%Y%m%d)
 - env BUILD_DIR_BASE=`pwd`/build/float make MORE_CFLAGS="-DBUILD_DATE='\"'$BUILD_DATE'\"'" defconfig all