I just made a fresh clone of the latest commit (because I hit this problem after pulling my current clone) and get a 'Task cannot be serialized' error in the final build step.  I tried 'make clean' to no avail.  My output is below.
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    LINK usr/bin/julia-release-webserver
    JULIA usr/lib/julia/sys.ji
Warning: replacing module Base
Task cannot be serialized
 in anonymous at no file
 in load_now at util.jl:231
 in load_now at util.jl:245
 in require at util.jl:174
 in process_options at client.jl:175
 in _start at client.jl:233
at /home/ian/repo/julia/base/sysimg.jl:269
 in load_now at util.jl:231
 in load_now at util.jl:245
 in require at util.jl:174
 in process_options at client.jl:175
 in _start at client.jl:233
 in load_now at util.jl:256
 in require at util.jl:174
 in process_options at client.jl:175
 in _start at client.jl:233
Note: this error is usually fixed by running 'make clean'.
$ make clean
$ make
    CC src/jltypes.o
    CC src/gf.o
    FLISP src/julia_flisp.boot
    FLISP src/julia_flisp.boot.inc
    CC src/ast.o
    CC src/builtins.o
    CC src/module.o
    CC src/codegen.o
    CC src/interpreter.o
    CC src/alloc.o
    CC src/dlload.o
    CC src/sys.o
    CC src/init.o
    CC src/task.o
    CC src/array.o
    CC src/dump.o
    CC src/toplevel.o
    CC src/gc.o
    LINK usr/lib/libjulia-release.so
    PERL base/pcre_h.jl
    PERL base/errno_h.jl
    PERL base/os_detect.jl
    PERL extras/glpk_h.jl
    PERL extras/julia_message_types_h.jl
    CC ui/repl.o
    CC ui/repl-basic.o
    LINK usr/bin/julia-release-basic
    CC ui/repl-readline.o
    LINK usr/bin/julia-release-readline
    LINK usr/bin/julia-release-webserver
    JULIA usr/lib/julia/sys.ji
Warning: replacing module Base
Task cannot be serialized
 in anonymous at no file
 in load_now at util.jl:231
 in load_now at util.jl:245
 in require at util.jl:174
 in process_options at client.jl:175
 in _start at client.jl:233
at /home/ian/repo/julia/base/sysimg.jl:269
 in load_now at util.jl:231
 in load_now at util.jl:245
 in require at util.jl:174
 in process_options at client.jl:175
 in _start at client.jl:233
 in load_now at util.jl:256
 in require at util.jl:174
 in process_options at client.jl:175
 in _start at client.jl:233
Note: this error is usually fixed by running 'make clean'.
I am using CentOS 6 and had no problems prior to my latest pull.