update README to require Ubuntu 16.04 at a minimum

Summary:
Drop instructions for building on Ubuntu 12.04, 13.10, and 14.04.  The oldest
Ubuntu platform we support these days is 16.04.

Our Travis CI configurations do still test building on Ubuntu 14.04 for now,
but it is not a vanilla 14.04 installation.  We upgrade gcc to 4.9 and CMake to
3.2 in order to build folly on this older Ubuntu version.

Reviewed By: yfeldblum

Differential Revision: D6780815

fbshipit-source-id: 0d27cdbb928d32c1527623c8862d50ce2215dd48

README.md

@@ -96,13 +96,7 @@ export LIBRARY_PATH=$BOOST_ROOT/lib:$LIBRARY_PATH
 ./configure --with-boost=$BOOST_ROOT/lib
 ```
 
-#### Ubuntu 12.04
-
-This release is old, requiring many upgrades. However, since Travis CI runs
-on 12.04, `folly/build/deps_ubuntu_12.04.sh` is provided, and upgrades all
-the required packages.
-
-#### Ubuntu 13.10
+#### Ubuntu 16.04 LTS
 
 The following packages are required (feel free to cut and paste the apt-get
 command below):
@@ -119,6 +113,7 @@ sudo apt-get install \
     libdouble-conversion-dev \
     libgoogle-glog-dev \
     libgflags-dev \
+    libiberty-dev \
     liblz4-dev \
     liblzma-dev \
     libsnappy-dev \
@@ -139,17 +134,6 @@ sudo apt-get install \
     libdwarf-dev
 ```
 
-#### Ubuntu 14.04 LTS
-
-The packages listed above for Ubuntu 13.10 are required, as well as:
-
-```
-sudo apt-get install \
-    libiberty-dev
-```
-
-The above packages are sufficient for Ubuntu 13.10 and Ubuntu 14.04.
-
 In the folly directory, run:
 ```
   autoreconf -ivf
@@ -159,10 +143,6 @@ In the folly directory, run:
   sudo make install
 ```
 
-#### Ubuntu 16.04 LTS
-The packages listed above for 13.10 and 14.04 are sufficient for installation,
-and the build commands remain the same.
-
 #### OS X (Homebrew)
 
 folly is available as a Formula and releases may be built via `brew install folly`.